This invention relates generally to the field of data communication isolation transformers and, more specifically, to isolation transformers with improved common-mode signal rejection.
Pulse transformers are used extensively in networking. In data communication based on differential signaling, transformers are utilized to provide balanced data transfer over copper cables. Such transformers also perform the function of impedance matching between dedicated drivers and the impedance of the cable. These applications require the transformer to have broadband characteristics, so that the bandwidth of the useful signal will not be limited or distorted by the transformer.
Due to the demands of the combining high package density and low cost, transformers in networking interface modules are typically wound on small toroids. The construction is usually based on bifilar windings, where the primary and secondary windings are wound together. That is, the primary and secondary transformer windings lie adjacent to each other and are wound about the toroidal core together. This results in good coupling, minimizing leakage inductance. One notable disadvantage of this construction is the inherent high interwinding capacitance. A result of this capacitance is that common mode noise, which is superimposed on the differential signals, passes through the transformer and enters the I/O area where it can be radiated by the attached cables, causing electromagnetic interference (EMI). To reduce EMI, some means of common mode attenuation, such as common mode chokes, must be provided.
Typically, high performance magnetic modules are constructed using two small wound toroidal transformers and one or two common mode inductors within a package. The design packaging goal in applications of multiple twisted pair ports, when the ports (such as RJ45 connections) must be placed close to each other, is for transformers to follow the density of the port placement and to be positioned sideways, adjacent to each other. This construction leads to two parallel rows of toroidal components within the package. A disadvantage of this construction is that due to the placement of the toroids and resulting loose wire termination, there is an increased likelihood of cross talk.
It is therefore an object of this invention to provide a data communications transformer with better common mode rejection from the transformer itself, without the need for external common mode filtering.
In accordance with the present invention, a data communication isolation transformer is provided that uses an xe2x80x9cExe2x80x94Exe2x80x9d or xe2x80x9cdouble-Exe2x80x9d core structure. The core has two E-shaped sections made of a material of relatively high magnetic permeability. When assembled, the two sections are located adjacent to, and in contact with, each other so as to form two flux paths through the core. The extending portions of each E-shaped section face each other such that the core has a center portion and two outer portions. Each of the flux paths through the core is through a different one of the outer core portions, and both are through the center portion. Since the center portion therefore supports twice the magnetic flux as the outer portions, it is preferable to have a center portion which has a cross-sectional area that is twice the cross-sectional area of either of the outer portions.
Mountable on the center portion of the core is a non-conductive bobbin, which fits snugly over the surface of the center core portion. The bobbin serves as a surface upon which the coils of the transformer may be wound, without the obstruction of the various portions of the core. The primary winding lies adjacent to the surface of the bobbin, and further from the bobbin surface is the secondary winding. This arrangement of primary and secondary windings allows an electromagnetic shield to be located between them. Thus, wrapped about the bobbin, between the primary and secondary windings is a shield of conductive material. The shield is electrically grounded to either the xe2x80x9cchassisxe2x80x9d side or the xe2x80x9clogicxe2x80x9d side of the apparatus in which the transformer is to be used. The shield attenuates common mode noise within the transformer, and may be either a thin piece of conductive material, such as copper foil, or a thin copper wire wound about the bobbin in the space between the primary and secondary windings. The shield may also consist of two separate layers (i.e. two pieces of conductive material or two wound wires) between the primary and secondary windings, one of which is grounded to the xe2x80x9clogicxe2x80x9d side of the apparatus, while the other is grounded to the xe2x80x9cchassisxe2x80x9d side.
The bobbin may be made of a non-conductive material such as paper tubing and, after placing the primary and secondary windings and the shield on the bobbin, the transformer core is assembled with the bobbin located on the center core portion. The entire transformer structure is then located within a standard IC package, with the leads of the primary and secondary windings, and the ground path for the shield, electrically connected to the mounting pins of the IC. This allows easy circuit board fabrication, mounting the transformer on the circuit board in the same manner as any other ICs. Since common mode attenuation is provided by the EMI shield of the transformer, it is not necessary to use a separate common mode choke, which would otherwise typically be located within the IC package. This significantly reduces the number of components in the IC package.